Gravity surveying is one technique in modern exploration for mineral and petroleum commodities. For example, detection of geophysically significant subsurface anomalies potentially associated with ore bodies or hydrocarbon deposits can be made using gravity surveying techniques since the existence of gravitational anomalies usually depends upon the presence of an excess or deficit mass associated with the deposit. The presence of the deposit can be associated with a gravitational anomaly of the body. The variation in gravity due to the presence of a body of ore can be 0.00002% of the normal Earth gravity field, for example. This relatively small effect is normally measured in units of milli gals (mGal), which is the unit for the free air and Bouguer gravity field measurements (1 mGal is equivalent to 10−5 m/s2).
Many gravitational measurements have been made using instruments of the LaCoste/Romberg type that are essentially ultrasensitive spring balances detecting a small difference in weight caused by the gravity anomaly. The measurements are subject to a wide variety of environmental influences, and measurements should be performed relative to a standard point that is used regularly during the survey as a fixed reference for removal of drifts in the instrument.
In addition, some large scale geophysical prospecting has progressed towards gradiometry. In principle, measurement of a gradient of a gravity field over a known baseline allows accelerations due to motion of the platform itself to be cancelled out. Gravity gradients are the spatial derivative of the gravity field, and have units of mGal over distance such as mGal/m. The standard unit of gravity gradiometry is the Eötvös (E), which is equal to 10−9/s2 or a tenth of a mGal over a kilometer (e.g., gradient signatures of shallow Texas salt domes are typically 50-100 E).
One type of gradiometry that has been used is three-dimensional Full Tensor Gradient (3D FTG) technology. FTG technology was developed by the US Navy and later adapted to the Oil & Gas industry to complement seismic technology and provide an independent method of imaging underwater salt and basalt areas, for example. Thus, gravity gradient measurements were originally collected using marine vessels or large ships to survey oceans. For example, using Bell Geospace's Marine-FTG™ technology, gravity gradient data collected from ocean surveys can be used to delineate subsalt and sub-basalt structures, predict bases of salt, and map other areas for valuable minerals.
Later, as gravity surveying moved on land, fixed-wing aircraft were adopted for data acquisition through airborne surveys. For example, a fixed-wing aircraft, such as a Cessna Grand Caravan, could be modified for airborne testing and evaluation, such as used within Bell Geospace's Air-FTG® technology. The success of airborne gravity gradient surveying to cover wide onshore areas with high precision data acquisition has attracted the interest of the mining industry. However, while such data is very useful for mineral exploration, there continues to be a desire to produce higher quality data.
A Full Tensor Gradiometer (FTG) includes three distinct modules known as gravity gradient instruments each measuring components of differential curvature. By processing outputs of the FTG, useful quality control measures and insight into a level and color (spectral shape) of gradiometer noise can be determined. When analyzing survey data, noise can be caused by environmental changes such as temperature and pressure and may be evident when studying output maps of the data.